1. Field of the Invention
The present invention relates to a backlight unit and a liquid crystal display, and in particular to a backlight unit consisting of red, green, and blue (RGB) light emitting diodes (LEDs) on a planar surface.
2. Description of the Related Art
Liquid crystal displays (LCDs) are used in a variety of electronic devices, including notebook and desktop computer monitors, mobile phones, car navigation systems, and TVs. In the conventional design, backlight units provide the main light source for LCDs. Direct backlight and edge-light technologies are employed, both of which use a cold cathode fluorescent lamp (CCFL) as the light source, particularly a white light source.
FIGS. 1A and 1B are schematic exploded views of a conventional direct backlight unit 10a and an edge lighting unit lob for a liquid crystal display. In FIG. 1A, the direct backlight unit 10a comprises at least a light source 11, a dispersion sheet 12a, a light-control sheet 12b, a reflector 13, and a frame 14. The direct backlight unit 10a locates the light source 11 between the light-control sheet 12b and the reflector 13. The light source 11 produces light, which is then reflected by the reflector 13 and diffused through the dispersion sheet 12a, providing desired color merged by the light-control sheet 12b. The direct backlight unit 10a is generally suitable for multicolor or dot matrix LCDs with larger screens.
In FIG. 1B, the edge lighting unit lob has at least one light source 11′ located along the edge of the light guide panel 17. A prism sheet 15 and a dispersion sheet 16 are disposed above the light guide panel 17. A reflector 13′ is located behind the light guide panel 17. Generally, the edge lighting unit lob provides less expensive lighting, especially for smaller LCDs. In addition, the edge lighting unit 10b offers a thinner package with lower power consumption, however, usually with lower light intensity.
Obviously, either size or brightness of the direct backlight 10a or edge lighting backlight lob is limited by the size of CCFL. Thus, other types of light source are considered to replace the CCFL. It is known that light emitting diode (LED) backlights offer a brighter and longer operating life of 50,000 hours minimum than other light sources such as CCFL with a life of 10,000 to 15,000 hours or electroluminescent (EL) with a limited life of 3,000 to 5,000 hours. Thus, compared to the conventional backlights using CCFL or EL, LEDs are less expensive, last longer, and smaller in size, contributing to a more robust design, and provide a higher brightness-to-size ratio. LEDs provide a variety of colors such as red (R), green (G) and blue (B) and consequently, must be mounted in an array directly behind the display to provide white light. Also LEDs are more compact than CCFL. If LEDs can be arranged to provide the brightest, whitest light, LEDs can also have the potential to provide excellent light source for all types of LCDs. However, a slight difference in arrangement of the LEDs can adversely affect the color and brightness of the display due to the combination of various colors. Hence, there is a need for a specific arrangement of the LEDs that can provide true white light in a backlight unit.